Conventionally, acid and hydrogen peroxide, base and hydrogen peroxide, or an oxidation-reduction pair are employed as etching solutions for etching semiconductors. For example, a mixture of sulfuric acid and hydrogen peroxide and a mixture of ammonia and hydrogen peroxide are well known. FIG. 17 is a sectional view for explaining an etching method using a conventional etchant. In the figure, reference numeral 1 designates an n type AlGaAs layer. An n.sup.+ type GaAs layer 2 is disposed on the n type AlGaAs layer 1. A photoresist film 3 is disposed on the n.sup.+ type GaAs layer 2. Reference character d designates an etching depth from the upper surface of the GaAs layer 2, and reference character .DELTA.d designates an over-etching depth by which the n type AlGaAs layer 1 is etched.
FIG. 18 is a sectional view for explaining an etching process in production of a high electron mobility transistor (hereinafter referred to as HEMT) using a conventional etchant. In the figure, the same reference numerals and characters as those in FIG. 17 designate the same parts. Reference numeral 4 designates a GaAs substrate. An intrinsic type (hereinafter referred to as i type) GaAs layer 5 is disposed on the GaAs substrate 4. An i type InGaAs layer 6 is disposed on the GaAs layer 5. The n type AlGaAs layer 1 is disposed on the InGaAs layer 6. The n.sup.+ type GaAs layer 2 is disposed on the AlGaAs layer 1. A drain electrode 7 and a source electrode 8 are disposed on the n.sup.+ type GaAs layer 2.
The threshold voltage V.sub.th of this HEMT is given by ##EQU1## where .phi.B is the Schottky barrier height, W is the thickness of the AlGaAs layer 1, N.sub.D is the donor concentration in the AlGaAs layer, .DELTA.E.sub.c is the energy discontinuity of the conduction band between AlGaAs and GaAs, and .epsilon. is the dielectric constant of AlGaAs.
In FIGS. 17 and 18, when a solution of phosphoric acid, hydrogen peroxide, and water mixed in a volume ratio of 3:1:50 is used as an etchant, the etching rates of the n.sup.+ type GaAs layer 2 and the n type AlGaAs layer 1 by this etchant are about 30 nm/min. Therefore, if this etchant is used for formation of a gate recess of the HEMT shown in FIG. 18, the recess depth d has a variation .DELTA.d.
In the formula (1) representing the threshold voltage V.sub.th of the HEMT, since the thickness W of the AlGaAs layer depends on the recess depth d, the thickness W varies as the recess depth d varies as described above, resulting in a variation in the threshold voltage V.sub.th that causes unstable characteristics of the HEMT.
FIG. 31 is a sectional view of a heterojunction bipolar transistor (hereinafter referred to as HBT) for explaining an etching process using a conventional etchant. In the figure, reference numeral 23 designates an n.sup.- type GaAs layer. A p type AlGaAs base layer 22 is disposed on the n.sup.- type GaAs layer. An n type AlGaAs emitter layer 21 is disposed on the base layer 22. An etching mask 24 is disposed on the emitter layer 21. In addition, reference characters d.sub.1 and d.sub.2 designate etching depths.
When a solution of ammonia, hydrogen peroxide, and water mixed in a volume ratio of 5:1:100 is used as an etchant, the etching rates of the n type AlGaAs layer 21 and the p type AlGaAs layer 22 by this etchant are about 90 nm/min. Therefore, the etching time must be controlled to stop the etching at the surface of the p type AlGaAs base layer 22. However, control to the level of one atomic layer is very difficult because of the variation in the etching rate. There is a great possibility that the etching front will stop at the depth d.sub.2 before reaching the surface of the p type AlGaAs base layer 22 or will stop at the depth d.sub.1 after etching a portion of the base layer 22.
When the conventional etchant is employed for the base surface exposing step in the production of an HBT, i.e., etching of the n type AlGaAs emitter layer 21 to expose the surface of the p type AlGaAs base layer 22 for electrical contact, the etching depth varies as described above. In this step, the base layer 22 is unfavorably over-etched as shown in FIG. 31 because the surface of the base layer 22 must be completely exposed for electrical contact.
FIG. 10 is a diagram for explaining a relationship between the etching for exposing the base layer and the external base resistance of the HBT. In the figure, reference numeral 11 designates an emitter electrode, numeral 13 designates a base layer, numeral 14 designates an emitter layer, and numeral 15 designates a base electrode.
The base resistance R.sub.B of the HBT shown in FIG. 10 is given by ##EQU2## where Z.sub.E is the emitter length and .rho..sub.B is the resistivity.
The external base resistance given by the second term of the equation (2) increases because the base layer 13 is over-etched, whereby the maximum oscillation frequency f.sub.max of the HBT is reduced.
As described above, since the conventional etchant has no selectivity for different materials or different properties of a material, if it is employed in production of heterostructure devices, precise control is difficult.